The goal of this project is to elucidate the function of huntingtin protein, mutation of which underlies Huntington's disease (HD). These mutations are expansions of a polyglutamine repeat motif (coded by CAG trinucleotide repeats in the mRNA) within the protein, making HD one of several known "glutamine repeat disorders". In this group of neurodegenerative diseases, an expansion of a polyglutamine repeat in a number of different proteins confers the various disease phenotypes. However, in spite of the relatively restricted patterns of differential cell death in HD and the other disorders, the gene products are widely distributed in both central nervous system and peripheral tissues, and both their normal functions and mechanisms of disease are largely unknown. This study will examine the function of huntingtin through three Specific Aims. First, analysis of the interaction between huntingtin and transcription factor MEF2 will be performed. In our preliminary work, we showed that the N-terminus portion of huntingtin specifically interacts with transcription factor MEF2. In this study the effect of expansions of the polyglutamine tract on the strength of MEF2-huntingtin interaction will be assessed. The domains of interaction will be studied by deletional analysis of the respective cDNA's. The effect of huntingtin protein with variable glutamine repeats on MEF2-dependent transcription will then be determined. Distribution of MEF2 proteins will be analyzed in transgenic and human HD brain by immunoflourescence. An emerging body of evidence supports roles for MEF2 as a calcium-dependent survival factor that protects neurons from apoptotic cell death. To determine whether MEF2 protects neurons from huntingtin toxicity, MEF2 levels will be manipulated in striatal cells by over-expression or RNA interference, and neurons will be scored for standard hallmarks of neuronal death. Identification of such molecular mechanisms in the pathogenesis of polyglutamine disorders may help identify possible targets for future therapeutics.